Teller Effect Research Articles

The Pseudo Jahn-Teller effect and NBO analysis for untangling the symmetry breaking in the planar configurations of M2X4+ (M = Si, Ge and X = Cl, Br, I): effect on electronic structure and chemical properties.

The Pseudo Jahn- Teller effect is a significant tool for evaluating molecular distortion and symmetry breaking. The PJT effect associated with NBO analysis can be a powerful method for studying the structural properties variations arising from D2h → C2h distortions. The theoretical studies on Si2X4+ and Ge2X4+ radical cations have been rare. The calculations have shown that C2h non-planar structures are more stable than planar structures with D2h symmetry. The [Formula: see text] PJTE problem of M2X4+ compounds is a result of the coupling between the ground B3u state and the exited B1u state in the Qb2g direction causes. Also, the difference in M and X atoms can affect the PJT instability of compounds. The findings of this work show that the energy gap between the ground and excited states that have D2h symmetry decreases from M2Cl4+ to M2I4+ and increases from Si2X4+ to Ge2X4+. In fact, there is a significant relationship between instability of high-symmetry configurations, geometric parameters, electron delocalization, chemical hardness, electronegativity, electrophilicity index, and PJT stabilization energies. These results may serve to evaluate the distortion of similar systems. The structures of Si2X4+ and Ge2X4+ are optimized by LC-BLYP, M06-2X, and B3LYP methods with def2-TZVPP basis set in GAMESS software. The details of the excited states of compounds are studied by the TD-DFT method. NBO analysis for planar and non-planar structures is carried out at B3LYP/def2-TZVPP level by the NBO 5. G program that demonstrates HOMO, LUMO, ED, bonding and antibonding orbital occupancies, bond order, and E2.

Preparation of long cycle lifespan spinel LiMn2O4 cathode material by a dual-modified strategy

Spinel LiMn2O4 has been identified as a cathode material with extensive application potential for Li-ion batteries. However, during cycling, the Jahn−Teller effect and Mn dissolution have a substantial impact on its specific capacity and cycle span. Herein, combining Fe and Cr elements co-doping with single crystal truncated octahedron morphology is proposed to solve these problems. The Fe−Cr co-doping markedly restrains the Jahn−Teller effect, and promotes the development of crystal and selective growth of crystal surfaces in spinel LiMn2O4. The high exposure (111) surfaces of the truncated octahedron reduce the dissolution of Mn, and a tiny proportion of (110) and (100) surfaces increase the diffusion channels of Li+. HAADF-STEM characterization indicates that the Fe and Cr elements largely replace the Mn atoms at the octahedral 16d site. Because of these advantages, the optimal LiFe0.05Cr0.08Mn1.95-xO4 (LFCMO-8) delivers excellent electrochemical performance with 119.6 mAh·g−1 discharge capacity and 78.1% capacity retention after 500 cycles at 1 C. At comparatively higher current densities of 20 C and 30 C, the superior capacity retention ratios of 79.8% and 87.3% are still achieved after 2000 cycles. Even at 40 C, the LFCMO-8 sample can still reach 98.8% capacity retention rate after 2000 cycles. After 1000 cycles at 5 C, the LFCMO-8 sample still maintained a capacity retention of 45.4% at high temperature (55 °C). ICP-OES test combined with a V-shaped tube device indicates that the Mn dissolution of LCFMO-8 only accounted for 11.8% of LiFe0.05Mn1.95O4 (LFMO). This study serves as a meaningful reference for the preparation of high-performance LiMn2O4 cathode materials.

Structural instability and poorly defined phase transitions in rare-earth dodecaborides RB12 (R=Ho-Lu) at intermediate temperatures

The single-crystal structures of YbB12, TmB12, and LuB12 were studied in the temperature range of 88–293 K, those of HoB12 and ErB12 – in the range of 88–500 K using high-resolution X-ray diffraction data in order to correlate the structure changes with the changes in the physical properties of the dodecaborides under study. The lattice deformations caused by the cooperative Jahn – Teller effect were studied. A method is proposed for approximating the temperature dependences of atomic displacement parameters (ADPs) from lattice sites using the extended Debye or Einstein models. The break points in the temperature dependences of ADPs in combination with nonmonotonic changes in the lattice parameters near the critical temperatures Tc indicate phase transformations revealed from diffraction data. The lattice instability and the rearrangement of the phonon spectrum near Tc are accompanied by the observed changes in the physical characteristics. A sensitive structural diagnostic tool was created to detect implicit phase transitions, quantum critical points and quantum instabilities of various nature that lead to the appearance of anomalies in the physical properties of crystals.

Density functional study of structural, electronic and magnetic properties of new half-metallic ferromagnetic double perovskite Sr2MnVO6

In this paper, a new half-metallic (HM) double perovskite compound is predicted with the simultaneous presence of ferromagnetism and polar distortion. The structural, electronic and magnetic properties of Sr2MnVO6 (SMVO) are calculated by density functional theory (DFT) with both generalized gradient approximation (GGA) and GGA + U approaches, where U is the on-site Coulomb interaction parameter. Different orderings of B (B′) cationic sites in A2BB′O6 double perovskite structure are evaluated, including rocksalt, columnar and layered arrangements for cubic, monoclinic and tetragonal crystal structures. It is found that the most stable ordering is obtained when B and B′ are placed in a layered type ordering for a tetragonal crystal structure with I4/m space group, which is confirmed by phonon calculations. The B-site ordering of the Mn3+ and V5+ ions in a layered configuration leads to ferromagnetically coupled magnetic moments of 4.17 µB at Mn site and 0.23 µB at V site. Finally, SMVO is found to be a half-metallic ferromagnetic (HM-FM) compound with a band gap of 0.65 eV in a spin down channel with off-centered displacement of V atoms in the octahedral cage (second order Jahn -Teller effect) which can cause ferroelectricity. Therefore, SMVO is predicted to be a polar HM material and a promising candidate for multiferroic property with potential application in spintronics.

First-Principles Investigation on the Electronic and Mechanical Properties of Cs-Doped CH₃NH₃PbI₃.

Methylammonium lead iodide, CH3NH3PbI3, is currently a front-runner as light absorber in hybrid solar cells. Despite the high conversion efficiency, the stability of CH3NH3PbI3 is still a major obstacle for commercialization application. In this work, the geometry, electronic structure, thermodynamic, and mechanical property of pure and Cs-doped CH3NH3PbI3 have been systematically studied by first-principles calculations within the framework of the density functional theory (DFT). Our studies suggest that the (CH3NH3)+ organic group takes a random orientation in perovskite lattice due to the minor difference of orientation energy. However, the local ordered arrangement of CH3NH3+ is energetic favorable, which causes the formation of electronic dipole domain. The band edge states of pure and Cs-doped CH3NH3PbI3 are determined by (PbI6)− group, while A-site (CH3NH3)+ or Cs+ influences the structural stability and electronic level through Jahn–Teller effect. It has been demonstrated that a suitable concentration of Cs can enhance both thermodynamic and mechanical stability of CH3NH3PbI3 without deteriorating the conversion efficiency. Accordingly, this work clarifies the nature of electronic and mechanical properties of Cs-doped CH3NH3PbI3, and is conducive to the future design of high efficiency and stable hybrid perovskite photovoltaic materials.

Specific Heat Investigation in Iron-Based Superconductors

We have proposed a mean field theoretical model in the coexistence of superconductivity and Jahn–Teller (JT) effect for the study of specific heat in iron-based superconductors in the presence of an external magnetic field. The superconducting (SC) gap and lattice strain energy expressions are calculated using Zubarev’s technique of double-time electron Green’s function and solved numerically. The specific heat jump at the critical temperatures are observed. Keeping fixed the SC transition temperature at 28[Formula: see text]K for the iron pnictide [Formula: see text] like superconductors, the coefficient of electronic specific heat [Formula: see text] at low temperatures is determined to be 5.75[Formula: see text]mJ/mol[Formula: see text]K2 in the pure SC state and 4.76[Formula: see text]mJ/mol[Formula: see text]K2 in the coexistence state of SC and JT effect which are in agreement with some experimental results. The effect of the external magnetic field on the specific heat is studied.

Topological effects in low-lying electronic states of linear N2H2+ and HBNH+ associated with onset of bending

ABSTRACTThe current decade has seen illuminating and extensive study of interplay of topological effects such as Renner–Teller and Jahn–Teller (JT) effects in small molecules. Study of such effects in HCNH molecule showed an interesting feature that for slightly bent system, a pair of JT conical interactions (CIs) appear only for certain nonplanar configuration, in contrast to its appearance in CS configuration (molecular plane) for C2H2+. Moreover, since the feature of appearance of JT-CIs in two 2Σ+ states of HCNH in some bent configuration may be critically associated to the abundance of HNC in interstellar spaces, as advocated recently by Das and Mukhopadhyay (J. Phys. Chem. 117, 8680 (2013)), the interest in such topological studies have remained relevant. In this article we have investigated such topological effects for slightly bent N2H2+ and HBNH+. Interestingly, the appearance of JT-CIs are, for HBNH+, in molecular plane while for N2H2+, in certain nonplanar configuration, in sharp contrast to the effect in the otherwise similar pair of molecular systems, C2H2+ and HCNH.

Magnetoresistance Scaling and the Anisotropy of Charge Carrier Scattering in the Paramagnetic Phase of Ho0.8Lu0.2B12 Cage Glass

The transverse magnetoresistance of Ho0.8Lu0.2B12 dodecaboride with a cage glass structure is studied at low (2–10 K) temperatures. It is demonstrated that the isotropic negative magnetoresistance in this antiferromagnet is dominant within the broad temperature range near TN ≈ K. This contribution to the total magnetoresistance is due to the scattering of charge carriers by nanoclusters formed by Но3+ ions, and it can be scaled in the ρ = f(μ2effH2/T2 ) representation. It is found that the magnetoresistance anisotropy above (about 15% at 80 kOe) is due to the positive contribution, which achieves maximum values at the magnetic field direction close to H ║ [001]. The anisotropy of the charge carrier scattering is interpreted in terms of the cooperative dynamic Jahn−Teller effect at В12 clusters.

Electric Field Control of the Valence‐Tautomeric Transformation in Cobalt Complexes

In this work a single valence‐tautomeric cobalt complex placed in a dc electric field has been examined. Along with the interaction with the electric field, the model employed accounts for intramolecular transfer of the excess electron as well as vibronic and exchange interactions. To obtain the energy spectrum of the valence‐tautomeric complex in the electric field, the vibronic problem of the pseudo‐Jahn–Teller effect has been solved. The vibronic (hybrid) wave functions and energy levels were used to calculate the temperature and field dependence of the molecular magnetic moment, as well as the polarization and shape of the absorption band in the near‐IR range, which is related to the light‐induced transfer of the electron from the catecholate ligand to the metal ion. It is predicted that the application of a dc electric field leads to electric polarization and a significant change in the magnetic and spectroscopic properties. The unique possibility of controlling electrically the magnetic state and optical and polarization characteristics of a valence‐tautomeric molecule is predicted.

Enhanced Anisotropy in Tetragonalized (Cu,Co)Fe2O4 Particles via the Jahn–Teller Effect of Cu2+ Ions

We studied the Jahn–Teller (JT) effect of Cu2+ ions on the tetragonalization of CuFe2O4 and (Cu,Co)Fe2O4 particles. Sub-micrometer-sized CuFe2O4 and (Cu,Co)Fe2O4 particles were synthesized via coprecipitation and flux methods, followed by a heat-treatment process. From the X-ray diffraction patterns, all particles were found to be cubic spinel after flux treatment. The tetragonalization process was carried out by a heat-treatment process in air, where the temperature was varied between 700 °C and 900 °C, followed by furnace cooling. We confirmed that the ideal temperature for the tetragonalization of both CuFe2O4 and (Cu,Co)Fe2O4 is 900 °C. The obtained single-phase tetragonal spinel-structured CuFe2O4 and (Cu,Co)Fe2O4 particles indicate tetragonal distortion as a consequence of the JT effect of the Cu2+ ions. The results of saturation magnetization suggest that most of the divalent ions are in the B sites. The lengthening of the c-axis due to the JT effect increased the coercivity of the CuFe2O4 and (Cu,Co)Fe2O4 particles during the cubic–tetragonal phase transformation. The tetragonal (Cu,Co)Fe2O4 particles showed a saturation magnetization of 26 emu/g and a coercivity of 2200 Oe.

Enhanced room-temperature MR and TCR in polycrystalline La0.67(Ca0.33−xSrx)MnO3 ceramics by oxygen assisted sintering

Polycrystalline La0.67(Ca0.33-xSrx)MnO3 (LCSMO, x = 0, 0.03, 0.06, 0.09, 0.12, and 0.15) ceramics were prepared by sol-gel method. Microstructures, electrical/magnetic transport properties, temperature coefficient of resistance (TCR) and magnetoresistance (MR) behaviors, were all investigated. X-ray diffraction (XRD) results showed that all samples crystallized in single phases without detectable secondary phases. At Sr doping (x) above 0.12, crystal structures transformed from orthorhombic phase (space group of Pnma) to rhombohedral phase (space group of R3̅c). As x content further rose, metal-insulator transition temperature (Tp) increased and maximum values of TCR and MR decreased. Further data revealed that TCR and MR peaks were controlled by Sr doping level x. In addition, room temperature MR and TCR of LCSMO ceramics were enhanced under oxygen assisted sintering. At Sr doping x = 0.09, the value of Tp attained room temperature (301.8K), and the maximum values of TCR and MR reached 10.5% K−1 and 43.0%, respectively. These properties were related to combination of double exchange (DE) and Jahn−Teller effects. LCSMO materials may have promising future use in photoelectric (uncooled bolometer or infrared detectors) or magnetic devices (uncooled magnetic sensors) at room temperature.

An ab initio investigation of non-adiabatic couplings and conical intersections among the lowest five electronic states of the NO3 radical

ABSTRACTIn order to explore spectroscopic properties of a floppy molecule, it is necessary to compute accurate diabatic potential energy surfaces (PESs) around the Franck–Condon region of nuclear configuration space. In such cases, it is desirable to include the non-adiabatic coupling terms (NACTs) through Jahn–Teller (JT) and pseudo Jahn–Teller (PJT) effects as well as other accidental conical intersections (CIs) prevailing among the electronic states of the molecule. In this work, we investigate NACTs among the five lowest doublet electronic states of NO3 radical [ and 22B2)], in which all those effects (JT, PJT and CIs) have prominent contributions. The study is carried out in terms of normal modes (Qis) of NO3 and the CIs (JT and accidental) are located in a plane of nuclear configuration space of two such coordinates (Qi and Qj). We have calculated ab initio adiabatic PESs and NACTs of those five states of NO3 at the MRCI level as functions of pairwise Qis. Further, to explore the nature of a given CI in terms of normal coordinates contributing to NACTs within a particular plane Qi-Qj, we have performed CP-MCSCF study computing analytic NACTs and characterised their contributions along various Qks.

The Distortive Nature of Temperature – A Symmetry Analysis

AbstractA new approach to resolve and quantify the effect of temperature on both the average, as well as the instantaneous, structure of generally symmetric molecules is presented. The method employs the efficient continuous symmetry measure, which is shown here to be capable of separating temperature‐induced instantaneous disorders from other sources of distortion, such as the Jahn−Teller effect. We demonstrate the potential of the method by studying two highly symmetric molecules: adamantane and the cyclopentadienyl anion and their cationic states. Our results show that although the distortive effect of temperature is significant, it does not mask other distortion effects. Symmetry analysis provides means to quantify the extent of the distortion in each case, and is investigated from a statistical point of view. A major experimental and theoretical challenge is to resolve the Jahn−Teller effect from other environmental effects, such as solvent, solid matrix, concentration, etc. Our results provide a step forward in this direction.

Magnetoinduced structural, Jahn–Teller, dielectric, and transport effects in La0.875Sr0.125MnO3

The temperature and magnetic field dependences of elastic moduli and resistance are studied in a La0.875Sr0.125MnO3 single crystal. In addition to the metal–insulator transition, the structural transition from the cooperative J–T strongly distorted orthorhombic phase to the charge ordering phase is studied at TJ–T = 150 K in magnetic fields of up to 2 T. Results show it is possible to control the acoustic parameters in the vicinity of TJ–T via magnetic fields.

Jahn–Teller and coupled Jahn–Teller/Renner–Teller effects in the calculation of adiabatic-to-diabatic transformation angle for the lowest three 2A′ states of NH2 (NHH)

We report here on the first study of topological effects for the NHH system, as carried out by treating simultaneously the two dominant effects of this system, namely, the Jahn–Teller (JT) effect and the Renner–Teller (RT) effect. Both the effects were treated rigorously as demanded by the Born–Oppenheimer approach. No approximations were made and in those cases where convergence was required, it was achieved by including the required number of states. The study concentrates on calculating the privileged adiabatic-to-diabatic transformation (ADT) angle γ12, along closed contours, which is the only needed angle to carry out the ADT in the case of relatively low energies. For this purpose, three coupled A′-states are usually considered and only in the last two extreme cases, where the area in configuration space becomes relatively large, namely 15–35 Å2, we had also to include an A″ state (the second Δ-state), a situation that enforces the more elaborate (JT/RT) effect. In this paper, we also report on potential energies as calculated along the above-mentioned contours. Among them are considered the energies associated with the two adiabatic Δ-states, 1A′ and 1A″, of different symmetry and therefore are responsible for the RT effect. These states are expected to be degenerate along the collinear axis. It was revealed that these states, in contrast to the Renner theory, are not degenerate along a finite interval of the collinear axis at the vicinity of the JT conical intersection (JT-CI). In other words, the JT-CI annihilates the RT degeneracy along this interval.

Ultrasonic Anomaly Near the Charge Ordering Transition in Sr-Doped Nd0.3La0.2Ca0.5−x Sr x MnO3 Manganites

Sr doping in the charge-ordered compound Nd0.3La0.2Ca0.5−xSrxMnO3 has been systematically studied to examine its effect on ultrasonic velocity and elastic moduli as well as magnetic and electrical transport properties. DC electrical resistivity, ρ and AC susceptibility, χ′ measurements showed all samples exhibit metal-insulator (MI) behavior accompanied by ferromagnetic-paramagnetic (FM-PM) transition where the MI transition temperature, TMI and FM-PM transition temperature, TC increased with Sr content indicating the enhancement of double-exchange mechanism. Analysis of the resistivity change with respect to temperature, dlnρ/dT−1 versus T indicates onset of charge-ordering (CO) state where its CO transition temperature, TCO decreased with Sr content indicating weakening of the CO state. On the other hand, both absolute longitudinal and shear velocities as well as elastic moduli measured at 80 K increased significantly with Sr doping indicating improvement in elastic properties, which is suggested to be due to the increase in formation of ferromagnetic domains. A longitudinal velocity anomaly characterized by a slope change around the vicinity of TCO was observed for all samples. The longitudinal elastic anomaly is attributed to the Jahn–Teller (JT) effect of Mn3+ ions where analysis of the anomaly using the mean-field theory suggests involvement of the JT effect in the samples, which transforms from dynamic to static type with decreasing temperature. The elastic anomaly shifted down from 222 K (x=0) to 205 K (x=0.05) indicating that the static JT effect was weakened with Sr content.

Synthesis, X-ray Structure, Magnetic Resonance, and DFT Analysis of a Soluble Copper(II) Phthalocyanine Lacking C−H Bonds

The synthesis, crystal structure, and electronic properties of perfluoro-isopropyl-substituted perfluorophthalocyanine bearing a copper atom in the central cavity (F(64)PcCu) are reported. While most halogenated phthalocyanines do not exhibit long-term order sufficient to form large single crystals, this is not the case for F(64)PcCu. Its crystal structure was determined by X-ray analysis and linked to the electronic properties determined by electron paramagnetic resonance (EPR). The findings are corroborated by density functional theory (DFT) computations, which agree well with the experiment. X-band continuous-wave EPR spectra of undiluted F(64)PcCu powder, indicate the existence of isolated metal centers. The electron-withdrawing effect of the perfluoroalkyl (R(f)) groups significantly enhances the complexes solubility in organic solvents like alcohols, including via their axial coordination. This coordination is confirmed by X-band (1)H HYSCORE experiments and is also seen in the solid state via the X-ray structure. Detailed X-band CW-EPR, X-band Davies and Mims ENDOR, and W-band electron spin-echo-detected EPR studies of F(64)PcCu in ethanol allow the determination of the principal g values and the hyperfine couplings of the metal, nitrogen, and fluorine nuclei. Comparison of the g and metal hyperfine values of F(64)PcCu and other PcCu complexes in different matrices reveals a dominant effect of the matrix on these EPR parameters, while variations in the ring substituents have only a secondary effect. The relatively strong axial coordination occurs despite the diminished covalency of the C-N bonds and potentially weakening Jahn-Teller effects. Surprisingly, natural abundance (13)C HYSCORE signals could be observed for a frozen ethanol solution of F(64)PcCu. The (13)C nuclei contributing to the HYSCORE spectra could be identified as the pyrrole carbons by means of DFT. Finally, (19)F ENDOR and easily observable paramagnetic NMR were found to relate well to the DFT computations, revealing negligible isotropic hyperfine (Fermi contact) contributions. The single-site isolation in solution and solid state and the relatively strong coordination of axial ligands, both attributed to the introduction of R(f) groups, are features important for materials and catalyst design.

Einfluss von H‐Brückenbindungen auf die Jahn–Teller‐Verzerrung in den Kettenstrukturen von 2,2′‐bipyMn(H2PO4)F2·H2O und 2,2′‐bipyMn(H2PO4)2F

AbstractIn the system 2,2′‐bipyridine/MnIII/HF/H3PO4/H2O two compounds with chain structures could be prepared and characterised by X‐ray structure analyses. 2,2′‐bipyMn(H2PO4)F2·H2O (1): monoclinic, twinned, space group P21/c, Z = 4, a = 6.7883(4), b = 10.9147(5), c = 17.8102(8) Å, β = 100.142(4)°, R = 0.0328. 2,2′‐bipyMn(H2PO4)2F (2): triclinic, space group P$\bar{1}$, Z = 2, a = 6.675(1), b = 10.715(1), c = 11.013(1) Å, α = 107.595(9)°, β = 90.994(9)°, γ = 95.784(8)°, R = 0.0252. Both compounds show chain structures with trans‐bridging dihydrogenphosphate ligands and bipy and two fluorine ligands for (1), or bipy, fluorine and an additional dihydrogenphosphate, respectively, for (2) in equatorial positions. Due to the pseudo‐Jahn–Teller effect, MnIII shows elongated octahedral coordination with ferrodistortive ordering along the chain direction. The distortion is remarkably higher in (1) than in (2). This is discussed in context with additional hydrogen bonds along the chain in (2).

Starker Pseudo‐Jahn–Teller‐Effekt in der Struktur von pyH[MnF2(C2O4)(H2O)2

AbstractThe first crystal structure of an oxalato‐fluoromanganate(III) was determined for pyH[MnF2(C2O4)(H2O)2]: monoclinic, space group I2/a, Z = 4, a = 7.2260(5), b = 15.2398(8), c = 9.916(7) Å, β = 98.22(5)°, R = 0.0297. The pyridinium salt crystallized from hydrofluoric acid solution at low temperature and in the dark. The structure of the anion shows octahedral coordination of MnIII with cis‐position of the fluorine ligands opposite to the bidentate bonded oxalate anion, and remarkable elongation of the H2O–Mn–OH2 axis due to a strong pseudo‐Jahn–Teller effect. The pyridinium cation is fixed to the oxalate anion by a bifurcated hydrogen bond. The anions are linked by strong hydrogen bonds O–H···F and O–H···O to form a 3D network.

Flattening a Puckered Pentasilacyclopentadienide Ring by Suppression of the Pseudo Jahn−Teller Effect

We report the theoretical prediction of flattening of the puckered Si5 ring by suppression of the pseudo-Jahn−Teller effect through coordination of two Mg2+ cations to the Si5H5− anion to make an inverse [Mg2+Si5H5−Mg2+] sandwich complex. The pseudo-Jahn−Teller (PJT) effect was suppressed through the OMO−UMO gaps increase in the resultant [Mg2+Si5H5−Mg2+] sandwich complex, as compared to the initial Si5H5− anion. It was the influence of two Mg2+ cations that caused the OMO−UMO gaps increase that made this type of PJT effect suppression work. In the three other complexes under the current computational study, namely, [Li+Si5H5−Li+], [Na+Si5H5−Na+], and [Be2+Si5H5−Be2+], the Si5H5− moiety remains nonplanar. We believe that if the Mg(Si5H5)2 solid compound were synthesized, it could have planar Si5H5− building blocks.